1. Technical Field
This invention relates generally to digital copiers (either black and white or color), and more particularly to dynamically determining the edges of text in images to improve image quality by applying appropriate processing operations.
2. Background Art
Original document images to be reproduced by digital copiers can be grouped into six primary image types: color text, color halftones, color continuous tones, neutral (black and white) text, neutral halftones, and neutral continuous tones. Often times there is a mix of these different image types on the same original document.
Various known image processing algorithms are used to convert the scanned digital information (typically 8-bit signals) to a printable format (typically one-bit to four-bit signals). In order to achieve the best image quality, different image processing algorithms need to be used as a function of input image type. Documents having mixed image types present a need to dynamically select appropriate algorithms as a function of image type within a document.
Recent systems for dynamically determining image types are disclosed in commonly assigned U.S. Pat. Nos. 4,856,075 and No. 4,862,283, which issued to Craig M. Smith on Aug. 8 and Aug. 29, 1989, respectively. Although these systems worked well for their intended purposes, the single bit thresholding algorithm which was selected when the algorithm determined the pixels were text pixels did not do as good a job on the edges of characters as does the present invention.
Also inherent in high speed color marking engines is a finite plane-to-plane registration problem within a document with respect to the laydown of different toners. Both these image quality issues are very noticeable in areas of input documents that contain black text.
A system that performs an edge enhancement on only black text within an image is used in the Canon CLC-500 Color Digital Copier. Canon's approach to detecting halftones is to count transitions over a two dimensional area. If the number of transitions is above a certain value, it is assumed to be halftone. This approach is susceptible to errors in small text, resulting in the screening of small characters. Further, the Canon approach cannot detect low frequency halftones. Large characters will also have their edges screened, and gray areas will tend to have dark edges around them.